Impacts of urban areas on regional air quality
Air pollution from cities can have impacts well beyond their geographical boundaries. Depending on the modelling scale, an urban area can be treated as a single point source with an urban plume impacting areas downwind. Part of the work to urbanise the UK Met Office Unified Model (UM) is to see how well the UM drives these interactions.
Surface level ozone concentration, showing the urban plume of London. In this case ozone depletion primarily due to urban concentrations of NOx have an impact downwind over distances in the order of 100 km.
Cities also affect the meteorology of surrounding areas by impacting thermo-dynamical processes at the regional scale. The urban surface stores heat during the day, which is emitted long after surrounding rural areas have cooled. This is the Urban Heat Island. The increased surface roughness of the urban area also has drag effects.
The CMAQ modelling system has been applied so far to several pollution events over the UK. The pollution episodes are typically characterised by high levels of pollutants such as ozone and its precursors in summer, NO2, SO2 and particles in winter. Urban areas and industrial stacks are the main sources of pollution. We investigated the impacts of both the London metropolitan area and point sources, which are spread out over the UK, on regional air quality for selected episodes. As an example the emissions from either the Greater London area or the point sources can be ‘switched off’ in the model.
The figure shows the impact of removing London (urban area within the M25 motorway) emissions on NOx concentrations at the ground surface, on 24 June 2001.
The impacts on regional air quality can be quantified by examining the variation in the distribution of pollution over the UK. Both the megacity of London and the major point sources have significant impacts on air quality at distances far from the sources of emission. The histograms show the contribution of the emissions from the main point sources and from London on NO2 (left) and SO2 (right) concentrations at the ground surface from 24 to 27 June 2001 (as an average over the UK).
Acknowledgements & collaborations:
This work is part of a project funded by UK Environment Agency.
Selected references:
Chemel, C., R. S. Sokhi, and A. Clappier, 2007. On the variability in the impacts of the London and Mexico City metropolitan areas on regional air quality. In Proc. of the 4th General Assembly of the European Geosciences Union, Vienna, Austria, Apr. 15–20, 2007.